Stereopsis

In the science of visual perception, stereopsis is the sensation that objects in space extend into depth, and that objects have different distances from each other. This sensation is much stronger than the suggestion of depth that is created by two-dimensional perspective. In humans, at least two mechanisms produce the sensation of stereopsis: binocular depth vision and (monocular) motion vision. In binocular depth vision, the sensation arises from processing differences in retinal images resulting from the two eyes looking from different, but similar, directions (binocular disparity). In motion vision, the sensation arises from processing motion information when the observer moves (e.g. optical flow, parallax). The sensation of stereopsis is similar in both cases. In research on depth vision, the term stereopsis is primarily used for binocular depth vision and not for the sensation of depth resulting from motion vision. Sometimes the term "relative depth" is used. This term emphasizes that it refers not to the distance to the observer, but to the mutual depth relationships of the perceived objects. If the meaning is clear from the context, the single word "depth" is also used instead of "relative depth." The word stereopsis comes from Greek stereós 'solid' and ópsis 'appearance, sight'. Together, these indicate seeing the outside of three-dimensional, "solid" objects. Binocular depth vision comes in two qualities: coarse stereopsis and fine stereopsis. Fine stereopsis plays a role in the recognition of shapes and objects, and coarse stereopsis in spatial localization.